Information recording media capable of optically recording digital information signals on information layers and reproducing information signals recorded on information layers with recording marks or recording pits (hereinafter called recording marks) have been gaining popularity due to the high level of quality and retrievability of the recording marks. Additionally, in recent years, DVDs (digital versatile disks) have been proposed as high density recording media that increase the volume of information that can be recorded on a single disk so as to better meet the demands of recording moving images and other high-volume information signals, and have become popular in that they meet the market demands. In addition, recordable DVDs (hereinafter called “writable DVD”) also are expected to provide increased reproduction and recording speeds, in the same manner as read-only media (e.g. CD-ROMs, etc.) containing, e.g., pre-recorded computer software programs. Incidentally, the constant linear velocity (CLV) system is advantageous in terms of increasing the density of recording and recording systems (hereinafter called the “CLV system” or the “CLV recording system,” including the ZCLV system, etc.) based on the CLV system have been developed and used in recording and reproducing devices used for recording and reproduction of writable DVDs.
On the other hand, as described above, writable DVDs are intended to increase the density of recording so as to record high-volume information signals, and, due to the higher recording density, there is a demand for maintaining records of recording information, such as write power and recording speed, etc. recommended by manufacturers of writable DVDs, information on the recording and reproducing devices used for recording on writable DVDs and/or compatibility of recording and reproducing devices with writable DVDs during recording, etc. For this reason, on the inner periphery of a lead-in area, which is located on the inner periphery of a data recordable area (Data Recordable Area), there is provided a recording management area (Recording Management Area), where records of recording management information are maintained, and a power calibration area (Power Calibration Area), which is used for keeping records of the irradiated laser power and recording speed, etc. used for recording in the data recordable area. The power calibration area is provided on the inside of the recording management area and, when the recording speed reaches a target value, it is used to make a record of information such as the laser power used to irradiate the information layer at the recording speed. This information is preserved as history for the purpose of learning about the irradiation power corresponding to the recording speed used when recording is resumed.
As described above, recording and reproducing devices perform recording using the CLV system and therefore, when recording is performed closer to the center, the rotational speed of the spindle motor rotating a writable DVD is increased. Thus, for example, while records of recording history information can be made at 4× recording speed in the power calibration area provided on the innermost periphery of a writable DVD, increasing the speed to 8× or 16× brings the rotation of the spindle motor to its limit and it becomes impossible to catch up and make records of recording history in the power calibration area.
Thus, taking into account the fact that merely providing the power calibration area on the innermost periphery is not sufficient to handle the demand for increased speed, Non-Patent Document 1 disclosed a configuration in which a new area, i.e. an outer drive area (Outer Drive Area), was provided on the outer periphery of the lead-out area as an area for maintaining records of recording history information that could not be handled in the inner disk test zone (Inner Disc Test Zone), with records of recording history information that could not be handled at high recording speeds kept in the outer disk test zone (Outer Disc Test Zone) of the outer drive area.
For example, FIG. 17 and FIG. 18 provide conceptual cross-sectional views of a conventional optical disk, with FIG. 17 depicting a recording area on the inner periphery of a DVD-R, and FIG. 18 showing a conceptual diagram of a recording area on the outer periphery of a DVD+RW. In the case of a DVD-R, an RMA area and an OPC area are provided further inward from the lead-in area. At the start of data recording, an information recording and reproducing device performs optimum write power calibration (OPC, Optimum Power Calibration) and, based on the calibration, determines the write power of the semiconductor laser. Here, “OPC” is an operation, wherein a predetermined test recording mark is written on an optical disk and the write power of the semiconductor laser is calibrated based on the beta value of the analog signal reproduced from the test recording mark. “OPC information” refers to OPC-related information including write strategies and write power conditions used during OPC. In addition, in the case of DVD-RAM and DVD+RW, OPC areas called disk test zones are provided on the inner periphery and on the outer periphery, but in FIG. 18, such an area is provided only on the outer periphery. After recording the entire data volume, the data recordable area reaches a radius of up to 58 mm, after which the lead-out area is recorded to a radius of 58.0˜58.05 mm and the disk test zone is recorded to a radius of 58.1˜58.3 mm.
Incidentally, substrates that transmit light used for recording and reproduction, and not only those used for DVDs, are formed by injection molding using polycarbonate and other resins. During such injection molding, a nozzle used for resin injection is placed near the inner periphery so as to allow the resin to spread across the entire surface of the mold. However, because the surface of the mold has a complicated geometry with tracks and embossing, etc., even though the process is designed to allow the resin to spread across the entire surface of the mold, the amount of resin supplied to the inner and outer portions of the substrate varies, resulting in inferior molding accuracy in the outer periphery portion. As a result, the molding accuracy of tracks and/or planar accuracy in the outer periphery portion of a disk is inferior in comparison with the inner periphery portion.
In addition, it is well known that in the past, the main component of the information layer of a writable DVD was an inorganic material in case of an erasable disk, and an organic dye in case of a write-once disk. Sputtering or vacuum vapor deposition and other so-called gas-phase deposition techniques are used for forming inorganic material-based information layers, and the method generally used for organic dyes is the so-called spin-coating technique in which a solution of an organic dye is applied to a rotating substrate in a dropwise manner. Because the spin-coating technique makes use of centrifugal forces generated by the rotation of the substrate, the organic dye solution is applied in a dropwise manner to the inner periphery of a substrate. For this reason, in write-once DVDs (so-called DVD-R, DVD+R) with information layers formed by spin-coating using an organic dye solution, the information layer at the outer periphery tends to have an insufficient amount of dye solution in comparison with the information layer on the inner periphery, resulting in inferior information layer quality.
Furthermore, the flatness of writable DVDs and other information recording media utilizing substrates produced by resin molding is lower than that of inorganic materials, such as glass. This creates a phenomenon called “surface wobbling”, wherein the surface of the disk moves up and down in the direction of the axis of rotation when the disk rotates. The surface wobbling phenomenon becomes more pronounced closer to the outer periphery and, in addition, as the speed of rotation becomes higher. Therefore, when information is recorded on the outer periphery at high speeds, there is a chance that surface wobbling may present a problem.
As explained above, the inner periphery is highly suitable as an area for keeping records of important information, while areas closer to the outer periphery are inferior from the standpoint of reliability. Therefore, the problem with the conventional configuration wherein, as disclosed in the above-described Non-Patent Document 1, an outer disk test zone (Outer Disc Test Zone) is newly provided on the outermost periphery of a writable DVD for recording information on the irradiation power corresponding to the recording speed, etc. (recording history information), is that it is impossible to ensure the reliability of such information.
In addition, the information area (Information Area), wherein an information signal can be recorded in a writable DVD, typically includes a power calibration area, a recording management area, a lead-in area, a data recordable area, and a lead-out area, and the problem with the conventional configuration is that it requires expanding the extent of the information area so as to compensate for recording past the outer periphery of the lead-out area.
[Non-Patent Document 1]
“DVD+R 4.7 Gbytes Basic Format Specifications version 1.2” (issued July 2003) (Page 48: 16.1 Physical Sector Numbers (PSNs) FIG. 22, Page 49: Table 7, see “Outer Disk Test Zone” in the “Outer Drive Area” column; Page 60: 21.3 Outer Disc Test Zone; and Pages 117˜119: Annex H Optimum Power Control and Recording Conditions).